Question

I am good at C++, but trying to understnd how singleton works. So I was looking into the code or article HERE .

If I will see the code;

class Singleton
{
private:
    static bool instanceFlag;
    static Singleton *single;
    Singleton()
    {/*private constructor*/}
public:
    static Singleton* getInstance();
    void method();
    ~Singleton()
    {instanceFlag = false;}
};

bool Singleton::instanceFlag = false;
Singleton* Singleton::single = NULL;
Singleton* Singleton::getInstance()
{
    if(! instanceFlag)
    {
        single = new Singleton();
        instanceFlag = true;
        return single;
    }
    else
    {return single;}
}

void Singleton::method()
{cout << "Method of the singleton class" << endl;}

int main()
{
    Singleton *sc1,*sc2;
    sc1 = Singleton::getInstance();
    sc1->method();
    sc2 = Singleton::getInstance();
    sc2->method();

    return 0;
}

In this above code that prints Method of the singleton class twice.
If we wanted to print the output twice, then why do we need singleton.
We can write like;

sc1->method();
sc2->method();

Why do need such a complicated code as written above.
One thing I noticed, instanceFlag is becoming true once condition satisfies with onject sc1 but when the object sc2 gets called then it goes to else part.

So, what exactly we are trying to do here?

Answer 1

You might want to read up wikipedia to understand what Singleton means

Essentially, for situations where you want to have only 1 object of a class at any time, you use such a pattern

In this code, Singleton::getInstance() is meant to return this 1 object of the class. The first time you call it, the object is constructed. Subsequent calls to Singleton::getInstance() will be returned the same object. instanceFlag tracks if the object has been instantiated.

Another hint is that the constructor is private, this means there is NO way to get an instance of the class except by the GetInstance() function.

p.s. I do agree with you though, this code is more convoluted than im used to.. It also (technically) has a memory leak .. the instance is never deleted

This is what i see more often

static Singleton* getInstance()  
{  
    static Singleton instance;  
    return &instance;  
}